The present invention relates to a means of compensating the magnetic field induced by the adjacent line in series of high intensity igneous electrolysis cells arranged transversely to the axis of the series. It applies particularly to series of igneous electrolysis cells for the production of aluminum by electrolysis of alumina dissolved in molten cryolite.
Industrial production of aluminum is often carried out by igneous electrolysis, in cells electrically connected in series, of a solution of alumina in cryolite brought to a temperature of the order of 950.degree. to 1000.degree. C. by the Joule effect of the current passing through the cell.
Each cell comprises a rectangular cathode forming a crucible, the bottom of which is formed by blocks of carbon fixed on rods of steel known as cathode rods which serve to evacuate the current from the cathode toward the anodes of the following cell. The anode system, also made of carbon, is fixed beneath an anode bus bar super-structure and is connected to the cathode rods of the preceding cell.
The electrolysis bath, that is to say the solution of alumina in cryolite, is located between the anode system and the cathode. The aluminum produced is deposited on the cathode. A layer of liquid aluminum about 20 cm thick is permanently kept at the bottom of the cathode crucible to provide a thermal fly-wheel effect.
Since the crucible is rectangular, the anode rods supporting the anodes are generally parallel to its large edges while the cathode rods are parallel to its small edges known as cell heads.
The cells are arranged in lines in a longitudinal direction or in a transverse direction depending upon whether their large side or their small side is parallel to the axis of the line. The cells are electrically connected in series, the ends of the series being connected to the positive and negative outputs of an electrical rectification and regulation sub-station. Each series of cells comprises a certain number of lines branched in series, the number of lines preferably being even so as to avoid needless lengths of conductors.
The electric current which travels through the various conductors such as electrolyte, liquid metal, anodes, cathodes and connecting conductors, creates large magnetic fields. These fields induce in the electrolysis bath and in the molten metal contained in the crucible so-called Laplace forces which are harmful to the proper functioning of the cell owing to the movements which they create. The layout of the cell and of its connecting conductors is designed so that the magnetic fields created by the different parts of the cell and the connecting conductors compensate each other. A cell having the vertical plane parallel to the line of cells and passing through the center of the crucible as its plane of symmetry is thus obtained. However, the cells are also subjected to interfering magnetic fields emanating from the adjacent line or lines. The term "adjacent line" means the line nearest the line under consideration and the term "field of the adjacent line" means the resultant of the fields of all the lines apart from the line under consideration.
In the following, the normal conventions will be adopted:
upstream and downstreams by reference to the direction of the electric current in the series, PA1 Bx, By and Bz, the components of the magnetic field along the axes Ox, Oy an Oz in a direct right-angled trihedron, whose center O is the center of the cathode plane of the cell, Ox is the longitudinal axis in the direction of the cell, Oy is the transversal axis and Oz is the vertical axis directed upwards, PA1 internal side of a cell, that which is situated toward the adjacent line and external side, that opposing the adjacent line.
Methods of compensating the magnetic field induced by the adjacent line have already been described in the past: Note U.S. Pat. No. 3,063,919, assigned to Pechiney, describes a demagnetizing loop device for attenuating the field of the adjacent line by making the return current come back for each line either beneath the line of cells or in the centre of the row of two lines of cells. Although this method is effective, it lengthens the conductors considerably.
U.S. Pat. No. 3,616,317 applies solely to series in which the cells are arranged in a lengthwise direction. It describes a device involving the positioning on the external surface of series arranged in two parallel lines, of a compensating conductor traversed by a direct current travelling in the opposite direction to that of the electrolysis current in the adjoining series and of equal strength at about 25% of the electrolysis current.
U.S. Pat. Nos. 4,072,597, and 4,090,930, assigned to Aluminum Pechiney, also describe methods of compensating the magnetic field of the adjacent line, but they operate cell by cell and not on the entire line and do not therefore arise from the same inventive idea.
However, the majority of these different prior art methods are unsuitable for compensating the magnetic field induced by the adjacent line or lines in the most recent installations where the intensity can reach and even exceed 200,000 amperes.
It would therefore be necessary to increase substantially the distance between lines in order to maintain an acceptable value at the field of the adjacent line. An unacceptable increase in certain expenses such as for ground, substructure amd length of the connecting conductors between lines of cells would thus result and diminish the profit on the investment allowed by the use of cells of high amperage.
The present invention relates precisely to a means of compensating the magnetic field of the adjacent line in series of very high intensity electrolysis cells arranged in a transverse direction.
It is essentially characterized by the installation, without modification of existing cells, of at least one auxiliary conductor, parallel to the Ox axis and situated in the plane of the bath/metal interface as near as possible to the pot that is to say to the external metal envelope of the cell. A direct current of an intensity selected so as to provide the desired compensation is passed into this auxiliary conductor in a suitable direction.